The image size + dpi and the file size for it are not related in a linear fashion. There are a number of options to reduce the file size whilst keeping the original image large. It depends whats in the image. In general tho images that have lots of random detail mean big and those with large areas of single colour are small.

We had a challenge once to make a map of a certain size and the file size was no bigger than 75Kb
http://www.cartographersguild.com/sh...op-WITH-PRIZES!!!

Here are the resulting entries.
http://www.cartographersguild.com/ut...gers_May08.htm

Some of these images are very nice maps and 75Kb is not a lot of memory for such an image. So there are options. I think that you need to post the kind of image you have that needs to be made a smaller file size. If your printing out on A4 sheets to stick together then you don't have one big map you have lots of smaller ones. This means that the computer does not need to allocate a lot of memory to print the whole map in one go. So I think the only issue is the disk space needed to ship or download the whole set of them.

Also, to cut a complex story short. Full commercial print is 400dpi +, good quality prints is about 300dpi, average or FAX quality is 200dpi, and below this looking at it in front of you your likely to see it pixelated.

I would also look into a little free app called posterazor (http://posterazor.sourceforge.net/) which takes one big image and breaks it up into A4 sheets with a little overlap so you can print and stick them together. Tho the PC needed to break up the image needs some RAM to cope with the big image, once broken up it will be easier for a smaller PC to print them a sheet at a time via a PDF of the pages.